The present invention relates to vehicles having four-wheel steering system, more particularly, to a rear-wheel steering system for a four-wheel steering vehicle.
Four-wheel steering vehicles in which the rear wheels are steered depending on the steering operation of the front wheels have been variously developed. Many of these vehicles are of the type wherein rear-wheel steering is carried out in dependency on signals respectively from a steering wheel turning angle sensor for detecting a turned angle of the steering wheel and from a vehicle speed sensor for detecting a vehicle speed. One example of this type of vehicle is disclosed in Japanese Utility Model Laid-Open Publication No. 73475/1986.
In a known four-wheel steering vehicle of the character described above, in general, a rear-wheel steering neutral position (rear-wheel steering angle zero) coincides with a point at which a signal from the steering wheel turned angle sensor is zero.
However, setting of the zero point of the signal from the steering wheel angle sensor is extremely difficult. The following example of the problems encountered will illustrate this difficulty more specifically. The zero point of the signal from the steering wheel angle sensor with respect to the neutral position of the front wheels (position for straight-ahead driving state) is initially set as the reference position Even when this is done, various factors cause this zero point to be inaccurate. For example, some deviation is caused by variation with lapse of time in the suspension alignment. As another example, an unbalance in the characteristics of the left and right tires are apt to arise. Examples of such tire characteristics are inflation pressure, abrasion loss, and tire uniformity. Due to such disturbing factors, some lateral force develops even when the front wheels are in the positions for straight-ahead driving state. Consequently, straight-ahead driving becomes impossible in this state. In such a case, a driver instinctively turns the steering wheel slightly, almost unconsciously, to restore the vehicle to its straight-ahead driving state. However, when the vehicle assumes this state, the point for sustaining the straight-ahead driving state by turning the steering wheel slightly becomes the actual steering wheel neutral position. This actual position is offset from the initially set zero point.
In such a driving state, while the front wheels are in their straight-ahead driving state, the steering wheel angle sensor generates signal indicating that the front wheels have been steered through an angle corresponding to the deviation from the initially set zero point. This signal causes the rear wheels to be steered according thereto. Thus, the straight-ahead driving is disrupted. As a result, a driver must further correct the steering wheel angle, so that the vehicle undergoes deviant running or bias running can be caused.